I have done this several times. Cut it at both ends, install new wire, tape to side of the harness and its done. My customers wouldn't pay several hundred in labor for a single wire, and it isn't worth the time.Why would you be worried about tearing out the entire wiring harness, because of just one wire?
Replace that one wire. End to end. All of it.
With the main plug on the firewall unplugged the headlight switch doesn't get power. But If I jumped power to it I am 100% sure it would pop the fuse because that brown wire ( that runs to all exterior lights ) is grounded. On the engine side main plug the brown wire pin reads 78omh and is grounded as well , with it unplugged. On the in cab side , from firewall plug to headlight switch it reads 0 omh and is also grounded.Does it blow the fuse if the main harness plug at the firewall is unplugged?
If it was just one short run of a single wire that wouldn't be much of an issue. But this wire runs from the headlight switch , behind the dash to the firewall plug , then to the power distribution box and finally out to all exterior lights. It could be shorted anywhere within that circuit so I'd have to rewire the whole circuit. I just suck at electrical stuff so it's hard for me to narrow it down any further. I'm sure it would be easy for someone that understands electrical diagrams better than me.Why would you be worried about tearing out the entire wiring harness, because of just one wire?
Replace that one wire. End to end. All of it.
With the main plug on the firewall unplugged the headlight switch doesn't get power. But If I jumped power to it I am 100% sure it would pop the fuse because that brown wire ( that runs to all exterior lights ) is grounded. On the engine side main plug the brown wire pin reads 78omh and is grounded as well , with it unplugged. On the in cab side , from firewall plug to headlight switch it reads 0 omh and is also grounded.
The brown wire is grounded. When you pull the switch out one click for running lights , pin b2 which is constant 12v , transfers power to the brown wire. It immediately blows the fuse. It's possible that I am not understanding the function of the switch correctly. But I'm pretty sure that's what is going on.Is the brown wire open or shorted when you pull the headlight switch?
The thing to remember when troubleshooting a lighting circuit, is that every bulb is a path to ground. So, if you’re simply checking for continuity between the brown wire and ground, you would need to remove every bulb that’s on that circuit.Ya it seems extremely unlikely that it's shorted on both sides.. but it sure seems like it.
So the weirdest thing happened. I cut the brown wire from the main firewall connector , on the engine bay side. I did that to eliminate everything that the brown wire runs to , and to see if maybe it was grounding out inside the connector. To my surprise it didn't pop the fuse and the running lights came on... I'm not sure how that is possible.. So then I supplied the engine bay side with a fused 12v jumper and it popped the fuse. It turns out the visor lights piggy back off the driver side running light and the positive side was grounding out under the battery tray. When I unhooked the visor to isolate it I disconnected it just short of where it was actually grounding out. I'm still not sure how the lights get power with that wire disconnected at the firewall , but I'm grateful it's fixed and ready to move on from this issue. Thank you everyone for the help!!
I couldn't figure out how to de-pin that connector lol . It wasn't as easy as the headlight switch side. I hate cutting in to a main harness but the wire was accessible and it was easy to repair it so I went that route.Glad you got it fixed! I think you could have de-pinned the connector instead of cutting the wire.
I found the short. It was right under the battery tray. I repaired it , I would never leave something like that even if it was working at the moment.The correct place to power the Visor lights would be from the plug intended for the cab running lights. On '92-'97 OBS trucks it's a 2 pin plug behind the passenger side kick panel near where the Door harness plug connects. On '87-'91 it's behind the driver side kick panel maybe also behind the parking brake assembly, I can't remember if you can access it with the parking brake assembly in place or if it has to be removed. Once you're connected you can run your wire up behind the A pillar trim and near the roof panel.
Hopefully you found the actual break in the insulation that is causing the short and isolated it. Otherwise if you still have the cut wire at the firewall and are relying on the short to feed the lights at the front, you might be out of luck one day when it decides to open up or over heat.